stags: Scala tags generator

Installation

Using Coursier:

coursier bootstrap co.pjrt:stags-cli_2.12:0.4.2 -o stags

If you want to use stags tag generation as a library, you can add it to sbt with:

libraryDependencies += "co.pjrt" % "stags_2.12" % "0.4.2"

Using Nailgun:

You can use Coursier to create a standalone cli for starting Stags with Nailgun like this:

coursier bootstrap --standalone co.pjrt:stags-cli_2.12:0.4.2 \
  -o stags_ng -f --main com.martiansoftware.nailgun.NGServer
stags_ng & // start nailgun in background
ng ng-alias stags co.pjrt.stags.cli.Main
ng stags --version

You can then create an alias for ng stags if that's still too much typing.

Caveats and tips:

  • You must call ng ng-alias after every restart of the nailgun server. You could create a script to do this
    • You could also simply make an alias in your terminal (ie: alias stags=ng co.pjrt.stags.cli.Main).
  • If you are running multiple Nailgun instances (for example, one for stags and one for scalafmt) you must run one of them in a different port.
    • In the above example, simply call stags_ng $new_port to run the stags ng instance in a different port. Then all ng calls need to have the flag --nailgun-port $new_port in them.
    • You could in theory simply feed both jars (ie: stags and scalafmt) into the same ng instance but beware this could cause classpath conflicts between the two (or more) jars.

Usage

stags ./

This will fetch all Scala files under the current directory. The tags file will be generated in ./tags. To place the tags file somewhere else, do:

stags ./ -o path/to/tags

Features

The two main differences between stags and a general ctags generator like Universal CTags is its ability to understand Scala code (with all its intricacies) and the ability to produce qualified tags.

Understanding Scala intricacies and static tagging them

What are static tags? Static tags are tags for "static functions". In the C world this means functions that can only be used in the file where they are defined; you could think of them as "private". Vim understand static tags and will match them first before anything else.

Static tags lend themselves nicely to private field and functions, so stags marks private statements and fields as static, while taking care of some Scala intricacies.

If a def/val/class/ect is private within its file, then it is static. If it is private for some large scope, then it isn't static. This means that if it is private[X] then we check if X is an enclosing object within the file. However, if X isn't an enclosing object in this file, then we mark it as non-static. For example

package org.example.somepackage.test

object X {
  object Y {
    private[X] def f = …
  }
}

object K {
  private[somepackage] def g = …
}

In this example, f would be static, but g isn't because g might be accessed from outside the file.

Other cases that are marked as static are:

  • constructor fields in classes (ie: in class X(a: Int, b: String, c: Boolean), a, b and c will all be static)
    • But non-static for the first parameter group of case classes (since those are accessible by default)
      • case class X(a: Int)(b: Int) <- a will be non-static, but b will be static
    • Any that are marked as "private" are static
  • the single field in an implicit class/case class
    • implicit class X(val x: Int) <- x is static
    • this is done because chances are that x will never be accessed anywhere but this file
  • all implicit things (val, defs, class, etc)
    • these things are rarely, if ever, accessed via their tokens

Qualified tags

A common pattern found when importing conflicting fields is to use them in a qualified form. For example:

import org.example.SomeObject
import org.example.OtherObject

SomeObject.foo(...)
OtherObject.foo(...)

In order to differentiate between the two, stags generates tags for all fields along with an extra tag that combines their parent with the tag itself. Note that stags never generates qualified tags for fields/methods in trait and class (only objects and package objects) since said fields/methods cannot be qualifiedly referenced.

Following code, by default, would produce three tags: Example, foo and Example.foo:

package object test {
  object Example {
    def foo(...)
  }
}

The depth of the qualified tags is controlled by --qualified-depth. Setting it to three (3) would produce a third tag test.Example.foo.

Vim support for qualified tags

Vim won't understand such a tag right off the bat. The following modification is required:

function! QualifiedTagJump() abort
  let l:plain_tag = expand("<cword>")
  let l:orig_keyword = &iskeyword
  set iskeyword+=\.
  let l:word = expand("<cword>")
  let &iskeyword = l:orig_keyword

  let l:splitted = split(l:word, '\.')
  let l:acc = []
  for wo in l:splitted
    let l:acc = add(l:acc, wo)
    if wo ==# l:plain_tag
      break
    endif
  endfor

  let l:combined = join(l:acc, ".")
  try
    execute "ta " . l:combined
  catch /.*E426.*/ " Tag not found
    execute "ta " . l:plain_tag
  endtry
endfunction

nnoremap <silent> <C-]> :<C-u>call QualifiedTagJump()<CR>